1. Field of the invention
The present invention relates to an expanded graphite sheet and production method thereof and, more particularly, to a heat resistant and asbestos-free graphite sheet which is utilized in gaskets or the like for sealing various joints of internal combustion engine and in other industrial purposes, and a production method thereof.
2. Description of the Related Art
Hithereto, joint sheets and beater sheets mainly composed of asbestos have been used as conventional gaskets or the like. Use of asbestos, however, has been increasingly regulated due to its harmfulness for the health.
Flexible expanded graphite sheet has been also known. This sheet is superior in sealing performance and heat resistance, but inferior in mechanical strength. Moreover, the sheet is difficult to handle in practical use and deficient in water resistance and oil resistance.
In substitution for the mentioned sheets, a sheet has been developed, in which filler is applied to aramid pulp or inorganic fiber and rubber-like elastic material is utilized as a binder. This sheet is produced by either a dry process or a wet process. However, this sheet is also inferior to asbestos sheet in mechanical strength, heat resistance and sealing performance.
Under such circumstances, improvement of the flexible expanded graphite has been attempted. For example, in the Japanese Laid-Open Patent Publication (unexamined) No. 124767/1990, it is disclosed that a rubber-like elastic binder is applied to expanded graphite particles to improve the bending breakdown resistance and oil resistance of the sheet, but has not advanced yet to the level of put a single compact thus formed into practical use. In the Japanese Laid-Open Patent Publication (unexamined) No. 288180/1987, another improvement is proposed in which an expanded graphite is impregnated with a thermosetting resin, and the resin is heated and dried to improve mechanical strength and liquid permeability. In this prior art, however, there arises a problem of losing flexibility which is the most important property, and moreover a problem of how to handle the product has not been solved yet. Furthermore, Japanese Laid-Open Patent Publications (unexamined) Nos. 71578/1985 and 72780/1988 disclose a method for mixing expanded graphite particles with reinforcing fiber or a binder. Various advantages of these prior arts are not sufficient to overcome the problems of brittleness and poor mechanical strength.